The present invention relates to a head cell apparatus used in wastewater treatment, and in particular, to an energy-efficient duct and methods used for conveying wastewater to be treated (i.e., influent) to the head cell.
One phase of wastewater treatment is separating xe2x80x9cgrit,xe2x80x9d which is high-density, inorganic, settleable particles, from the influent. Grit causes wear to downstream treatment equipment and, if it accumulates, loss of performance.
One type of apparatus used for separating grit from influent is referred to as a head cell. Other approaches to removing grit have involved the use of a horizontal mechanically rotating element (e.g., a paddle or propeller) that circulates the influent within a surrounding cylindrical tank to separate the grit from the influent and cause it to gather in an accumulating well. By way of contrast, head cells separate grit by a continuous hydraulic action and do not require any mechanically-induced motion. Head cells are also self-cleaning.
Using a mechanically rotating element is disadvantageous because the periodic nature of its rotation creates turbulence that tends to re-suspend finer grit. Also, larger objects that are typically found in an influent flow, such as rags, as one example, can accumulate and xe2x80x9cbridgexe2x80x9d operating areas in the well. In this case, such an apparatus must then be drained, cleaned and/or repaired, which results in decreased treatment efficiency and increased operating costs.
The hydraulic separation action in a head cell occurs through controlling the influent to flow at predetermined speeds and along a predetermined course, and does not require the use of chemicals. The influent enters at the periphery or rim of a funnel-like conical surface from a direction tangential to the rim, and then flows over and around the downwardly sloping conical surface, at least partially circling a centrally located opening. The flow conditions are determined such that a dynamic boundary layer is developed at the conical surface.
As the influent flows around the downwardly sloping conical surface, the grit is separated out onto the conical surface. At the same time, the remaining liquid, i.e., the effluent (which is relatively grit-free wastewater) is guided to flow out of the head cell through openings located at the outer periphery of the conical surface. In general, this effluent is channeled for further treatment downstream, e.g., as primary sludge.
At the same time, the separated grit moves downwardly along the sloping conical surface and through the opening for collection at a point beneath the opening. A head cell may have several individual conical surfaces or xe2x80x9ctraysxe2x80x9d that are vertically aligned with each other such that grit draining through the central opening in an upper tray also passes through similar central openings in all lower trays. In a typical head cell having vertically aligned or xe2x80x9cstackedxe2x80x9d trays, a greater working surface area is provided relative to the head cell""s footprint than for comparably sized equipment having a single chamber with a mechanically rotating element.
In some head cell installations, referred to herein as xe2x80x9cupward feed head cells,xe2x80x9d the influent is pumped vertically upward such that each of the stacked trays, in succession from a bottom tray to a top tray, receives an amount of the influent through a peripheral inlet. The energy requirement of this arrangement can be high due to the loss of influent velocity head and the necessity of additional head to generate a suitable velocity in the peripheral inlet. In many installations, however, available head is limited, making this arrangement impractical.
Some wastewater treatment installations were originally implemented without grit removal equipment positioned upstream of the primary sludge treatment equipment. Retrofitting such installations with grit removal equipment is desirable to eliminate or at least reduce the amount of grit in the primary sludge before it enters the primary treatment equipment. Given the floor space constraints in existing installations, head cell equipment is often favored because it has a far superior capacity to remove grit (as great as 10 times more) per unit area of the equipment""s footprint than the mechanically rotating element design. These retrofit installations, however, often have the same energy limitations that prevent use of an upward feed head cell.
New methods and associated apparatus are provided for operating a treatment apparatus, e.g., a head cell, with improved performance and efficiency. Influent is fed downwardly to a head cell, which decreases the head cell operating energy (i.e., head) requirement and increases efficiency. Because the new method requires less head, the head cell can now be used in situations with low available head that were previously restricted to mechanically rotating element equipment. Head cells provide superior performance over mechanically rotating element equipment by removing smaller size grit and removing a higher percentage of grit in all other larger size ranges.
A new energy-efficient passageway member or duct is provided that directs influent downwardly to distribute it at multiple levels of a head cell, from the top down. The new duct substantially minimizes head losses and preserves the required flow conditions for proper operation of the apparatus. The duct reduces and may eliminate, in some cases, the excess energy requirement for upward feed head cells in which the influent is pumped in an upward vertical direction.
According to some implementations, the duct directs or feeds a single flow from a higher level downwardly and into multiple flows at lower levels substantially without any head loss. Influent may be directed from a higher level to each of the multiple trays of a head cell that are positioned at lower levels without requiring an additional energy input, e.g., to pump the influent or to power a grit-removing mechanically rotating element. Within a given energy limit, a head cell provides superior performance because of its greater working surface area (i.e., the combined area of the multiple trays) per unit area of footprint than the working surface area of a mechanically rotating element system having a comparable footprint. The working surface area of a mechanically rotating element system is limited to a portion of the inner planar surface of the cylindrical tank.
In commonly encountered retrofit situations, the lower performance of mechanically rotating element equipment, sometime referred to as a gravity grit chamber, would prevent removal of fine grit from the system""s effluent before it enters other downstream processing equipment, e.g., primary treatment equipment. The remaining fine grit causes undesirable wear. In the same situations, however, a downwardly fed head cell removes substantially all of the fine grit.
The duct has an inlet end that is positioned higher than the outlet end such that influent will flow downwardly to the outlet end. The outlet is divided into multiple, individual nozzles that are vertically separated from each other. The nozzles terminate in openings or orifices through which a portion of the influent is directed to each corresponding individual tray of the head cell.
The outlet end of the duct is positioned to direct the influent flowing through the duct into the head cell from a direction generally tangential to the periphery of each tray.
The duct may have various sections along its length from the inlet end (i.e., where raw influent is received, e.g., from an open channel, to the outlet end (i.e., where the duct joins the head cell). The duct may have a first section in which the velocity of the influent flow is changed as necessary (e.g., by changing a cross-section of the first section), a second section that drops from a higher level to a lower level and has a constant cross-section, and a third section in which the influent is distributed into multiple nozzles, which may be at different levels.
With the duct, influent is conveyed from a source at a higher level to multiple lower levels, and the velocity of each multiple flow where it enters the treatment apparatus is maintained at a predetermined design velocity. As necessary, the duct is configured to initially change the influent velocity to match the design velocity, and the influent is then maintained at this design velocity throughout the remaining downstream length of the duct. Advantageously, the influent is conveyed from the source to the treatment apparatus substantially without any head loss.
Recitation of any aspect in this Summary of the Disclosure is not intended to imply that the aspect is an essential element. The Summary is instead provided to facilitate understanding of the following detailed description.